The present invention relates to multistage centrifugal pumps in general, and more particularly to improvements in multistage centrifugal pumps with follow-up diffusers or guide rings which are disposed downstream of the impellers and serve to direct the liquid medium into the next-following pump stages.
Centrifugal pump technology has developed certain standards for the design of the impellers and of the follow-up diffusers or guide rings in such fluid flow machines. A distinguishing characteristic of a guide ring is a flow path or passage with an increasing cross-sectional area in the direction of flow, and the guide ring can perform one or more functions including deceleration, deflection and/or return flow of the conveyed fluid medium. The neighboring passages of a guide ring are separated from each other by guide vanes or blades. The important parameters of such passages include their length and the ratio of the areas of their inlets and outlets. The length of the passage is normally between two and four times the width of the inlet and the area of the outlet is between 1.3 and 1.7 times the area of the inlet. The area of the inlet (also called inlet eye) equals the width of the inlet of the passage, as measured in the radial direction, multiplied by the width of the guide wheel as measured in the axial direction of the pump. The width of the inlet is an imaginary line extending orthogonally from the inner end portion of one blade to the next blade.
A further parameter which is normally considered in the design of multistage centrifugal pumps is the diameter ratio D5/D2 which is dependent upon the number of blades and the specific speed. Such ratio is normally between 1.2 and 1.5. D5 denotes the outer diameter of the guide wheel and D2 denotes the outer diameter of the impeller.
A drawback of many presently known multistage centrifugal pumps is that the outer diameter of the pump housing is too large and that it cannot be reduced without adversely affecting the efficiency of the pump.